Catholics believe that the one true priest is Jesus Christ, but that in His earthly life, Jesus gave a share in his priesthood to the 12 apostles. The apostles in turn ordained other men to share in the priesthood of Christ (2 Timothy 1:6), and the priesthood has thus been handed on through the authority of the Church for two millennia.
The priesthood is not a position of power, but of service (Matthew 20:25-28, John 13:2-20). The modern, secular world tends to view the priesthood from the perspective of politics and power, but Jesus specifically rejects such a view in the Gospels.
Priests carry on the ministry of Jesus through the sacraments and the preaching of the Gospel, they are called to strengthen the people of God so they might be the light of the world.